[task scheduler] Use repograph.UpdateWithCallback to refresh caches and add new jobs
[task scheduler] Use repograph.UpdateWithCallback to refresh caches and add new jobs

This should fix problems resulting from the repo update taking place
outside of the main loop.

Bug: skia:8850
Change-Id: I44c425d495de83051f2edce7893984130f0756d6
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/200920
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot